The problem is not your girlfriend's sexual orientation, it is how both of you deal with it.
If she wants to be with girls, then fine. This relationship clearly isn't enough for her. But if you let her walk all over you by "fooling around" with her friend, then that makes it seem like you don't care about her enough to continue a relationship.
Again, it doesn't matter if she is attracted to both sexes, but it matters how you deal with it. Accepting it is good, rejecting it isn't a great idea, but allowing her to do whatever she wants with her friend isn't a way of saying "I care a lot about this relationship and don't want you fooling around with other people."
I am homosexual. A lesbian, actually. If you have a problem with that, then it sucks for you. It's not your problem. It's not even a problem. I quite like it this way.